One of my all-time favorite plant galls is the appropriately named Translucent Oak Gall. The galls are produced by the gall-wasp, Amphibolips nubilipennis (family Cynipidae). They arise from a leaf vein on the lower leaf surface and measure around 1/2 - 3/4" in diameter. Their shape and color causes them to strongly resemble tiny, pink balloons or pink grapes hanging beneath the leaves of red, scarlet, and black oaks.
Slicing the succulent gall open will reveal a single wasp larva housed in a chamber in the center of the gall. The larval chamber appears to be resting on a white pedestal. The gall will have a single, round hole through the surface of the gall that leads to a tunnel to the larval chamber if the immature wasp has completed its development exited its gall-home. As with most oak galls, these galls can be appreciated free of fear for the health of the host tree